Barium oxide has been well known as a material that reacts with oxygen to thereby be converted into barium peroxide through an oxidation reaction shown in Chemical Formula 1, adsorbing oxygen during this process, and releasing oxygen through a reduction reaction under oxygen-free atmosphere. Due to these properties of barium oxide, barium oxide has been used in a process of producing oxygen in the early 20th century.
                              BaO          +                                    1              2                        ⁢                          O              2                                      ->                  BaO          2                                    [                  Chemical          ⁢                                          ⁢          Formula          ⁢                                          ⁢          1                ]            
However, since barium peroxide does not have thermal stability at a high temperature, barium peroxide tends to lose oxygen adsorptivity while a cycle progresses. In more detail, barium peroxide is a significantly thermally unstable material, such that a sintering phenomenon between particles thereof may be generated at a high temperature. Therefore, a size of the particle may be increased, and oxygen adsorptivity may be gradually lost as a process progresses. In order to prepare oxygen-selective adsorbent capable of maintaining adsorptivity, this phenomenon should be blocked. In addition, since barium peroxide easily reacts with another mineral or metal component due to excellent reactivity to thereby lose a binding property with oxygen, there is a need for making a structure capable of stably protecting a barium component (barium oxide) selectively adsorbing oxygen.
Various methods for solving this problem have been attempted, and among them, a method of fixing barium oxide to dolomite to increase a use-rate and reactivity has been disclosed in U.S. Pat. Nos. 3,773,680 and 3,903,010. In the method disclosed in U.S. Pat. Nos. 3,773,680 and 3,903,010, an oxygen-selective adsorbent was prepared by simply mixing barium oxide and a solid dolomite with each other to be used to produce forming method to pellet with high pressure.
It is disclosed in U.S. Pat. No. 4,092,264 that an oxygen adsorbent capable of increasing a use-rate of barium and having improved stability may be prepared by impregnating barium oxide with zirconia. A method of impregnating barium oxide disclosed in U.S. Pat. No. 4,092,264 is as follows. First, porous zirconia is fired at a high temperature to remove impurities, mixed with barium peroxide, and then the obtained mixture is heated to allow barium peroxide to be impregnated with zirconia. According to the patent, in the case of impregnating barium peroxide with zirconia by the above-mentioned method, thermal stability may be increased, and heat generated in an oxidation reaction with oxygen at the time of performing an oxygen producing process may be effectively stored and used at the time of performing a reduction process, such that efficiency of the process may be increased. Here, when barium is impregnated with zirconia at a level of 20% or less, stability may be maximally implemented, and the use-rate of barium may be increased.
According to the patents, a method of mixing or impregnating barium peroxide with a third substrate mainly in a dried state has been used. However, in the case of using the method of mixing barium peroxide with the third substrate, a non-uniform protective film is formed, such that a use-rate of barium oxide may not be high, and in the case of impregnating barium peroxide with the third substrate, since a material acting as the substrate is required in excessive amounts, it may be impossible to develop a material having high oxygen adsorptivity.
As another method, after barium oxide and a precursor of magnesium oxide (aqueous magnesium salt) are appropriately mixed in an aqueous solution, barium oxide may be captured in a framework of magnesium oxide by inducing evaporation of water to form precipitates and being subjected to a sintering process at a high temperature. However, in this method, barium oxide reacts with water during the sintering process to form barium hydroxide, which has low stability and high reactivity, such that it is not easy to perform the sintering process, and oxygen adsorptivity may be decreased after sintering.